1. Field of the Invention
This invention relates generally to devices for steam-cooking food in the home; and more particularly to a third-generation, countertop type, electrically powered apparatus that is very space-, energy- and time-efficient, self-enclosing, readily cleaned, safer than earlier units, and versatile in handling varying quantities and shapes of food.
2. Prior Art
(a) First Generation Steamers--Food steaming is of course well known, and until recently was performed by placing food on racks inside generally conventional stovetop cooking vessels. Such vessels generally took the form of large saucepans or the like, with loosely fitted gravity-closed covers; however, pressurized vessels were also used.
All such equipment may be regarded as the first generation of food steaming devices for home use. It had many disadvantages.
First, it occupied a burner on the stove which otherwise might have been used for some other dish. Secondly, it required heating substantially the entire cooking vessel to the temperature of boiling water, before steaming could begin. This was wasteful in terms of time, since the user had to wait for the whole vessel to come to temperature in addition to waiting for the food to cook thereafter; and also in terms of energy, since the entire heat capacity of the cooking vessel had to be filled with heat in order to vaporize a small amount of water.
In addition, large vessels amenable to heating on a stove burner to vaporize water were, at least will until quite recently, necessarily of metal--or other opaque or at best translucent materials. Such construction impaired the user's ability to monitor visually the progress of the cooking.
Moreover, when more than one level of food rack was employed, the user had to contend with maneuvering of the hot racks out of the vessels, and manipulation of the food out of the hot lower racks. Of course millions of people mastered the tricks eventually, but they all remember well the burned fingers that led to this mastery.
At one time, pressure vessels were in great vogue for cooking. The pressure cooker is a special case of the stovetop steamers discussed above, and has all the drawbacks already mentioned--but in exaggerated form, because of the massive walls and cover-attachment mechanisms required to resist high internal pressure.
At least for light foods of diffuse structure such as leafy vegetables, or food such as carrots whose shape provides a high ratio of surface area to cookable volume, the time and energy required to heat the thick walls of a pressure cooker make that device inefficient. For such foods a pressure cooker thus uses even more heat, and is even more difficult, hazardous and slow, than the other stovetop steamers whose drawbacks have been discussed above.
All those drawbacks inspired the development of electrically powered countertop steamers. Such units may be regarded as a second generation of food-steaming apparatus for home use.
(b) Second-Generation Steamers--One such early steamer has enjoyed great popularity and commercial success because of its space-efficient and extendable self-enclosing configuration. In that device, a square cast-aluminum water tray is embedded in a plastic base.
A circular channel is formed in the underside of the tray to receive a sealed electrical heating rod, and the edges of the channel are crimped against the rod at several points to hold the heating rod firmly to the tray. A sheet-metal reflector is fastened under the heating rod to reflect heat away from the plastic base, and a separate thermostat unit is secured below the reflector.
Although very successful, that system has certain disadvantages. A significant fraction of power consumed by the heater is radiated to the plastic base directly or by reradiation from the reflector, never reaching the water tray. Heat conduction to the tray is inefficient except at points where the heater is forced hard against the tray.
Before cooking can begin, the heat capacity of the entire tray must be filled with heat until much of the tray is at the boiling point of water; hence yet more heat is wasted to the plastic base by downward radiation from the tray. Furthermore, due to good conduction within the metal of the water tray, the edges of the tray are very hot--causing warping of the plastic base along some edges of embedment, and also posing some hazard to users and others from exposed hot surfaces.
The reason for embedment of the water tray in the base was apparently to avoid even greater hot-surface hazard. The embedded configuration, however, leaves the unit susceptible to water leakage into the base around the edge of the tray, and consequent possible electrical hazard. This possibility is aggravated by the previously mentioned warping of the base along some edges of the tray, opening what should be relatively tight seams between the two components.
In that same earlier cooker, a perforated square aluminum screen rests on a peripheral ledge of the water tray, passing water vapor upward and returning condensate to the tray. A user of the device places food upon the aluminum screen, above the water in the tray--and also provides a confined space for contact of the food with steam.
The user has four choices for provision of this confined space. First, the user can place a cover directly upon the edge of the screen, supported by the tray ledge. The cover is a unitary plastic article, square in plan, with nearly vertical sides roughly two inches tall.
Secondly, when the user wishes to cook a larger quantity of food than can be fitted between the cover and the screen, the user can interpose between the screen and the cover an integral plastic extender, also about two inches tall. The extender has neither floor nor ceiling, serving only to elevate the cover above the screen to accommodate the larger quantity of food--and in particular tall pieces of food.
Thirdly, if the larger quantity of food which the user wishes to cook is in the form of small or shallow pieces, the user may not wish to place the entire quantity in a single mound on the tray. In some cases the weight may be too much for the tray, and in some cases steam may not penetrate easily to the center of the mound.
To avoid both these problems the user may position a second screen just above the extender and below the cover, to act as an elevated shelf. That arrangement distributes the weight of the food between two screens, and also permits division of the food into two separate groupings for better access of the steam.
Fourth, for further versatility the user can acquire additional metal screens and plastic extenders, and add shelf-wise and dome-wise to the two-layer structures described in the previous paragraphs. For example, a three-extender lower chamber could be topped with a one-extender-plus-cover upper chamber; or with two extenders and two more screens used in the shelf mode.
Whether the cover is supported by the screen on the water-tray ledge, or by an extender, or by a second or further screen on the extender, steam produced by heating of water in the water tray--and rising through the screen or screens --is generally confined by the cover. As will be understood throughout this document, the confinement provided by such plastic pieces resting on the water tray or on other plastic pieces, or on screens, is not strictly airtight and certainly not a high-pressure seal, but similar to the previously mentioned gravity-closed saucepan covers in retaining the greater portion of the steam within the enclosure for contact with the food.
When present, the extender participates in providing this confinement. In any of these variant configurations, food positioned between the bottom screen and the cover is confined in contact with the steam and thereby cooked. The plastic cover and extender advantageously (but optionally) can both be made transparent for better visual monitoring of the cooking process.
Water condenses on the walls and underside of the cover, and on the surfaces of the food, and together with juices from the food runs down through and around the screen or screens into the water tray for reuse. Some of the combined water and food juices in contact with the screen surfaces reevaporates from those surfaces.
Evaporation of combined water and juices from the aluminum water tray, and reevaporation of such mixtures from the hot aluminum screen surfaces, form gummy and chemically adherent mixed deposits of minerals and food residues on the metal. These deposits sometimes also attack the metal.
The tray and screen surfaces thereby become very unsightly and sometimes weakened. The water tray is essentially nonrepairable and acounts for the major part of the cost of the entire apparatus.
In addition to other drawbacks mentioned above, the screens have no handles. Removing or otherwise handling them for cleaning is therefore awkward at best--especially when the screens are hot. Removal of the lower screen in particular is most readily accomplished by inverting the water tray, but this requires dealing with water that remains in the tray. The screen, however, obscures the user's view of water in the tray.
If the screens did have handles, additional problems would arise in cleaning of the handles or their attachments, or in the hazard of the hot handle surfaces. The screens, as well as the water tray, are relatively expensive parts of the apparatus simply because they are of aluminum.
Furthermore, the hot aluminum screens, with their sometimes disrupted or degraded surfaces, are in direct contact with the food. Very recent research reports have demonstrated a possible health hazard from use of aluminum in cooking. More specifically, what has been preliminarily shown is a correlation between at least one human disease, namely Alzheimer's disease, and abnormal concentrations of aluminum in the brain. The significance or causality of this correlation is not known, and certainly no part of the purpose of this document is to raise undue alarms in this regard; however, one may conclude that--at least when all other things are equal--aluminum may be best avoided in construction of cooking equipment.
Another commercially available product in the second generation of home cooking equipment will now be mentioned, although in many ways it is less advanced that the unit already discussed. This steamer has baskets that are round in plan, and made of plastic and metal in combination. This device is very inefficient in terms of countertop space usage and overall food capacity: a square has over twenty-five percent more area than the corresponding inscribed circle.
Moreover the combined-material baskets retain many or most of the same metal-cleaning problems, hot surfaces, and possible health hazards of the unit discussed earlier. Some cleaning problems are exacerbated by the seams and crevices where plastic and metal are joined; in fact, basic sanitation can be impaired by such structures as they age.
Worst of all, the steamer under discussion lacks provision for modular extension of the cooking volume. A single-volume cooker suffers badly from lack of versatility: if the volume is big enough for, say, four ample portions of food, then the amount of energy and time required to cook two such portions is needlessly long. On the other hand, if the volume is appropriate for two portions, then four people cannot be served.
The baskets of this particular second-generation unit, nevertheless, only hold food and do not confine the steam. A separate external domed enclosure--also round and rather massive--is required for that purpose.
Accordingly the apparatus is much more cumbersome in use, and when the cover is removed it consumes counter space or other kitchen space. Even if placed on its side it occupies a large amount of countertop space; and being circular it is likely to roll away, unless the user positions it thoughtfully with an eye to this possibility. Generally speaking the cover is just one more piece of equipment to handle, clean, juggle in use of the apparatus, store, protect against marring or other damage--and, of course, purchase.
The separate external cover imposes an artificial limit on the number of baskets that can be stacked together, and at the same time it imposes an artificial constancy on the volume of the enclosure. Hence, in the device now under discussion the fundamental objective of modularity in basket construction in the first unit discussed earlier--namely, accommodation of the amount of time and energy consumed to the amount of food cooked --is lost.
The device now under discussion has yet another group of drawbacks. Water condensed on the food basket, or on the food itself, runs down to a collection well around the outside of the water tray--rather than back to the tray for reuse. Hence the device recycles neither the water nor the heat required to elevate the water from tap-water temperature to just below the boiling point.
Accordingly the unit runs out of water more rapidly, requiring greater attention during use to refill the water tray before it runs dry; and also consumes more energy in heating the refill water. This device does, however, eliminate the problems associated with a large cast-metal water tray by employing instead a small-diameter die-cast crucible heater centrally mounted in a plastic water tray.
The crucible heater has some operating advantages in terms of the amount of water required to produce steam, and the fraction of heater power that goes into the water when the water level is very low. Unfortunately, however, the crucible heater is relatively expensive in comparison with other small cast heater types.
In the apparatus under discussion, a horizontal web portion of the water tray forms a long path for heat conduction from the heater to the peripheral portions of the device which a user is likely to touch--thus effectively isolating the heater thermally from the user-accessible portions. In such a configuration the water tray need not be guarded by a separate plastic rim, and the tray is extended into a lip which surmounts the edge of the electrical-enclosure base. This lip, however, is continuous with the peripheral collection-well structure mentioned above, preventing recycling of condensate and producing the concomitant disadvantages already described.
In yet a third commercial unit of the second generation of home steamers, the water-tray features are similar to those just described, but the baskets used are all plastic. The latter feature provides substantial improvement in amenability to cleaning.
Although the baskets of this third commercial unit can be stacked shelf-wise, this stackability does not provide any modular increase in cooking capacity. In this device too, the baskets only hold food and do not confine the steam.
Here the confinement is provided by a relatively tall upstanding oval wall that is unitary with the base, and an oval cover. In other words, in configuration this unit is like a covered pot. In effect this second-generation cooker actually emulates a first-generation stovetop covered saucepan or pot!
Food-containing baskets are positioned inside the external pot structure. Accordingly the apparatus is cumbersome and unpleasant to use, since a person must peer into, and reach down into, the pot to observe the progress of cooking, and to place or remove the baskets.
As in the second device already discussed, the external structure of this third device--i.e., the "pot"--imposes an artificial constancy on the volume of the enclosure. This arrangement obviates the fundamental objectives of modularity in basket construction.
In this third device, the cover is relatively shallow, and therefore can be stood on edge when not in use, for conservation of countertop space. Unfortunately, however, the cover is generally oval in plan. Unless placed securely on one of its longer edges, it may be susceptible to rolling away from the position where it is left.
From the foregoing discussion of three different commercial offerings in the second generation of home cookers, the reader will understand that prior artisans in this field have done little more than experiment willy-nilly with various modes of construction. Although various favorable features have appeared in one or another of these devices, these efforts have not been enlightened by any overall appreciation of the benefits or burdens of different structures; or in particular of interactions between the various features.
Accordingly the prior art in this area has failed to advance beyond a group of second-generation devices, all clearly improved over first-generation units but all also respectively flawed by very severe practical handicaps. The prior art has thus failed to provide a truly satisfactory overall configuration that is optimally compact, space and energy efficient, easy to clean, safe, and versatile.
Furthermore this has been the state of affairs in the field of home steamers for at least several years. The householder's need for such a unit has accordingly been long felt, but unmet.